This invention is concerned with the formation of fine-grained halide bodies. In particular, the present invention is concerned with the preparation of high strength halide bodies for use as optical components in infrared systems.
One of the more critical problems encountered in the development of high power infrared lasers is the development of laser windows which are highly transparent to laser radiation at 10.6 microns and at 3 to 5 microns. At the present time, considerable research effort has been devoted to the development of laser windows from the so-called covalent compounds consisting typically of II-VI compounds such as cadmium telluride, zinc telluride, and zinc selenide. The need for improved laser window materials, however, is well known. See, for example, F. Horrigan et al, "Windows for High Power Lasers" Microwaves, page 68 (January, 1969); M. Sparks, "Optical Distortion by Heated Windows in High Power Laser Systems," J. Appl. Phys. 42, 5029 (1971).
The need for improved laser windows is based on the extremely high laser power throughput required and the fact that laser windows constitute structural members. In order to maintain high throughput and minimize adverse optical effects, the amount of energy transferred to the window must be kept low. Laser beam energy can be transferred to the window in two ways: heating of the window caused by either bulk or surface absorption of the beam, or direct conversion of the beam energy to mechanical energy by brillouin scattering or electrostriction. This energy transfer produces several undesirable effects such as lensing and bire-fringence, which result in degradation of beam quality and polarization. In extreme cases, severe thermal stresses can be produced in the windows. These stresses which are further aggravated by the fact that the windows are mounted in a cooling clamp, may lead to fracture of the windows.
The low absorption coefficients of the halides make them outstanding candidates for optical components in infrared systems. The alkali halides exhibit low absorption from the near ultraviolet to beyond 10.6 microns, and the alkaline earth halides exhibit low absorption in the 2 to 6 micron region. Furthermore, because the temperature coefficient of the index of refraction and the coefficient of thermal expansion have opposite signs, the two effects tend to compensate optical path changes due to temperature, making these materials useful in applications in which heating by a laser beam is anticipated.
Halide crystals, however, have low yield strengths and are highly susceptible to plastic deformation. These mechanical properties of single crystal halides have limited their use as high power laser windows.
The outstanding transparency of the halide materials makes it very attractive to attempt to overcome their mechanical deficiencies. Halides can be strengthened without altering their optical properties by hot working of single crystals to produce fully dense polycrystalline materials.
Fine-grained polygonized halide bodies can be produced by pressing, rolling, or a combination of pressing and rolling. In my previously mentioned U.S. Pat. No. 4,118,448, I described a process for forming fine-grained halide bodies at low temperatures by use of a constraint technique. A constraining ring around the halide body applies a compressive hoop stress which inhibits cracking which would otherwise occur during hot working. This technique yields structures which are extremely fine-grained and which can exhibit yield strengths over an order of magnitude higher than the starting single crystal billet. A further advantage of this process is that under certain conditions (temperature, strain rate, initial crystal orientation) the optical properties of the fine-grained billet are identical to those of single crystal material.
While the constraint technique described in my copending patent application has many advantages, it does have a few disadvantages. First, a constraining ring or rings is needed for each billet. The ring may not be reused. Second, the starting halide crystal requires preparation so that it is cylindrical and so that it fits the constraining ring. Third, cracking of billets still occur at low temperatures and very high strain rates even when constraining rings are used. Fourth, when large billets are to be hot-worked, the use of large constraining rings can be cumbersome.